If I have two on-premises Citrix farms, can I migrate the management plane into Citrix Cloud, put some resources in Azure, and still aggregate the two on-premises farms together with the new cloud-based resources through Citrix Gateway Service? (I should know all this off-by-heart but I have been out of circulation for a year :-))

Yes, under Workspace Configurations there is a Sites tab for info on the on-prem CVAD site to be added so everything can be aggregated together

Yes